APPLICATION NOTE
4
Single Forward Convertor
Figure 3 shows the conventional forward converter circuit. You will first understand how this circuit
operates. When switch M1 is turned ON, current through the transformer winding starts building up. This
current is equal to the transformer magnetizing current and the load current. At this time, diode D1
conducts and D2 is reverse biased. In this stage of switching cycle, energy is transferred to the inductor
L3 and the load. When switch M1 is turned off, negative voltage appears across the transformer winding.
This negative voltage makes D1 reverse biased and D2 forward biased. The energy stored in L3 and C1
continues to supply the load while current continues to flow through D2. This circuit does not show the
RESET winding for primary magnetizing current. There are multiple methods for the same but these are
not included in this note.
Figure 3: Single Switch Forward Converter
The output voltage in this case depends upon the Duty Cycle and the transformer ratio. Output voltage
can be calculated using the following equation:
Vout = Vin * D *
Where
D is duty cycle
N1 and N2 are the number of turns of transformer windings
PWM Controller
General purpose offline PWM controller UC3843 is used in this application. This device is from a family of
control devices that provide the necessary features to implement off-line or dc-to-dc fixed frequency
current mode control schemes with a minimal external parts count. Internally implemented circuits include
